Postfix: Generic mapping not working

I have followed the guide "Howto: ISP-style Email Server with Debian-Etch and Postfix 2.3" (http://workaround.org/articles/ispmail-etch/) (postfix, mysql, dovecot, amavisd-new, clamav, spamassasin). I am using the latest postix 2.4.5 on a Ubuntu 7.10 amd64 server and everything works fine except for one thing -- the generic mapping.

I have, let say, a mylocaldomain.com which is my name of my local network and an external domain called mydomain.com.

The problem is that when I am sending an email, the recived mail get the headings:

Note that I have the line: smtp_generic_maps = hash:/etc/postfix/generic. In this "generic"-file I have tried many different mappings without success. It seems that this file is not read, I am not sure. Note also "receive_override_options = no_address_mappings" but this only effects the received mails and not when you send, or?

As far as I under stand is that when I am sending an email it goes first to Amavis on port 10024. Since the transport map for Amavis includes "smtp_generic_maps= ", the "generic"-file is not used at this stage. The email is then sent to port 10025 where Postfix take over to finally send the email. Since the transpost map for "127.0.0.1:10025" does NOT contain "smtp_generic_maps= " the generic map should be used, but it doesn't. Or?

I have one more question. Let say that some of you solve my problem and the generic mapping works! How can I do the following:
user.mylocaldomian.com myname.domain.com #(case 1)
user.mylocaldomian.com myname.otherdomain.com #(case 2)

I don't know if you understand what I am trying to say. Let say I host two domains on my server (my work mail and private mail). If I am the user on a local machine and want to send a mail using "myname.domain.com" I want the generic map to use "case 1" but if I instead use "myname.otherdomain.com", the "case 2" should be mapped.

I guess I don't really understand the generic mapping because I can not see how this can be done. So how do you do something like this?